Why must the base class be specified before interfaces when declaring a derived class?

public interface ITest
{
    int ChildCount { get; set; }
}

public class Test
{
}

public class OrderPool : ITest, Test
{
    public int ChildCount
    {
        get;
        set;
    }
}

The error says Base class 'Test' must come before any interfaces. Why is it necessary to extend the class first and then implement the inteface?